Functional fibers having high mechanical strength, high modulus of elasticity and a high heat resistance, for example, para-aromatic polyamide fibers, are widely employed as reinforcing materials for resin composites containing as a matrix, rubbers, epoxy resins or phenol resins, in industrial practice. These functional fibers are disadvantageous in that the bonding property to the matrix resins is low due to such a fact that the surfaces of the functional fibers exhibit a high smoothness, and the polymeric materials from which the functional fibers are formed exhibit a poor chemical activity, and thus the reinforcing effect of the functional fibers is not so high as expected from the high mechanical strength, for example, the high tensile strength, of the functional fibers.
It is known that spun yarns and stretch broken fiber yarns produced from the functional fibers have excellent bonding property to various types of matrix resins due to an anchor effect thereof provided from the fluffs present on or close to the peripheries of the yarns, and exhibit a good reinforcing effect on the matrix resins.
However, the spun yarns and stretch broken fiber yarns are formed from short length fibers produced by cutting or stretch breaking continuous filaments, and thus the resultant spun yarns and stretch broken fiber yarns exhibit a significantly lower mechanical strength, for example, tensile strength, than that expected from the mechanical strength of the original filaments. Therefore, the spun yarns and the stretch broken fiber yarns do not exhibit as high a reinforcing effect as that expected from the mechanical strength of the original filaments.
For the purpose of solving the above-mentioned problems of the conventional reinforcing yarns, a method in which functional groups for enhancing the bonding property to various types of the matrix resins are introduced into chemical structures of the polymers for forming the reinforcing fibers, and a method in which the stretch breaking length of the stretch broken fiber is increased to enhance the contribution of the mechanical strength of the original filaments on the mechanical strength of the resultant stretch broken fiber yarn, have been provided.
In the former method, however, the types of the functional groups to be introduced must be varied in response to the types of the matrix resins, and this necessity causes not only the productivity of the reinforcing yarns to be degraded but also the cost of the reinforcing yarns to be increased.
In the latter method, the contribution of the mechanical strength of the original filaments on the mechanical strength on that of the resultant stretch broken fiber yarn can be enhanced. However, as it is true that the fibers forming the yarn are short length fibers prepared by stretch breaking the continuous filaments, a reinforcing effect as high as that expected from the mechanical strength (tensile strength) of the original filaments cannot be realized.
Accordingly, a new type of reinforcing yarn in which the mechanical strength of a multifilament yarn is utilized with a high efficiency and the bonding effect to the matrix resins is satisfactory and the production cost is relatively low, is strongly desired.